U.S. patent number 10,735,978 [Application Number 16/409,532] was granted by the patent office on 2020-08-04 for multi-band cellular antenna system.
This patent grant is currently assigned to Quintel Cayman Limited. The grantee listed for this patent is QUINTEL CAYMAN LIMITED. Invention is credited to David Edwin Barker, Peter Chun Teck Song.
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United States Patent |
10,735,978 |
Barker , et al. |
August 4, 2020 |
Multi-band cellular antenna system
Abstract
An antenna system may include a first phase shifting network to
output a first set of component signals from a first component
signal having a first frequency, a second phase shifting network to
output a second set of component signals from a second component
signal having a second frequency, the first frequency being less
than the second, and an antenna array, the first phase shifting
network to impart a first tilt angle for the first set of component
signals, the second phase shifting network to impart a second tilt
angle for the second set of component signals, and a single
variable electrical tilt controller to control the first and second
phase shifting networks and being configured to maintain a ratio
between the first tilt angle and the second tilt angle, where the
first tilt angle is greater than the second tilt angle, and where
the ratio is less than one.
Inventors: |
Barker; David Edwin (Stockport,
GB), Song; Peter Chun Teck (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
QUINTEL CAYMAN LIMITED |
George Town, Grand Cayman |
N/A |
KY |
|
|
Assignee: |
Quintel Cayman Limited (George
Town, KY)
|
Family
ID: |
1000004967712 |
Appl.
No.: |
16/409,532 |
Filed: |
May 10, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190349783 A1 |
Nov 14, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62670488 |
May 11, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01Q
21/0025 (20130101); H01Q 5/50 (20150115); H01Q
1/246 (20130101); H04W 16/28 (20130101); H01Q
21/22 (20130101); H01Q 3/38 (20130101) |
Current International
Class: |
H04W
16/28 (20090101); H01Q 21/00 (20060101); H01Q
5/50 (20150101); H01Q 3/38 (20060101); H01Q
21/22 (20060101); H01Q 1/24 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion mailed in
corresponding PCT/US2019/031846 dated Sep. 10, 2019, 11 pages.
cited by applicant.
|
Primary Examiner: Miah; Liton
Attorney, Agent or Firm: Tong, Rea, Bentley & Kim,
LLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Patent
Application Ser. No. 62/670,488, filed May 11, 2018, which is
herein incorporated by reference in its entirety.
Claims
What is claimed is:
1. An antenna system comprising: an antenna array comprising a
plurality of antenna elements; and a feed network to: create signal
pairs from an input signal having at least a first component signal
of a first frequency and a second component signal of a second
frequency; impart frequency-dependent phase differences to the
signal pairs; and impart amplitude differences to the signal pairs,
the amplitude differences of each signal pair dependent upon the
frequency-dependent phase differences of the signal pair, the
signal pairs comprising antenna element drive signals of the
plurality of antenna elements of the antenna array, wherein the
antenna element drive signals provide a first array illumination
function for the first frequency and a second array illumination
function for the second frequency, wherein the second frequency is
greater than the first frequency, and wherein the second
illumination function has a greater amplitude taper compared to the
first illumination function.
2. The antenna system of claim 1, further comprising: a plurality
of splitters, to create the signal pairs from the input signal.
3. The antenna system of claim 2, further comprising: a plurality
of fixed line lengths, to impart the frequency-dependent phase
differences to the signal pairs.
4. The antenna system of claim 3, wherein at least one of splitter
ratios of the plurality of splitters or lengths of the plurality of
fixed line lengths are tuned to provide the greater amplitude taper
to the second illumination function as compared to the first
illumination function.
5. The antenna system of claim 1, further comprising: a plurality
of quadrature hybrid couplers, to impart the amplitude differences
to the signal pairs.
6. The antenna system of claim 5, wherein the antenna array
supports radio frequency (RF) signals over a range of RF
frequencies, wherein the range of RF frequencies includes the first
frequency and the second frequency.
7. The antenna system of claim 6, wherein each of the plurality of
quadrature hybrid couplers is tuned to a designated frequency
within the range of RF frequencies.
8. The antenna system of claim 7, wherein the designated frequency
comprises a mid-frequency within the range of RF frequencies.
9. The antenna system of claim 1, wherein the first illumination
function is to provide a first radiation pattern via the antenna
array, and wherein the second illumination function is to provide a
second radiation pattern via the antenna array, wherein the first
radiation pattern and the second radiation pattern provide a
substantially identical coverage footprint for a range of angles
from three degrees above the horizon to three degrees below the
horizon for a deployment of the antenna array.
10. A method comprising: creating, via a feed network, signal pairs
from an input signal having at least a first component signal of a
first frequency and a second component signal of a second
frequency; imparting, via the feed network, frequency-dependent
phase differences to the signal pairs; and imparting, via the feed
network, amplitude differences to the signal pairs, the amplitude
differences of each signal pair dependent upon the
frequency-dependent phase differences of the signal pair, the
signal pairs comprising antenna element drive signals of a
plurality of antenna elements of an antenna array, wherein the
antenna element drive signals provide a first array illumination
function for the first frequency and a second array illumination
function for the second frequency, wherein the second frequency is
greater than the first frequency, and wherein the second
illumination function has a greater amplitude taper compared to the
first illumination function.
11. The method of claim 10, wherein the signal pairs are created
from the input signal via a plurality of splitters of the feed
network.
12. The method of claim 11, wherein the frequency-dependent phase
differences are imparted to the signal pairs via a plurality of
fixed line lengths of the feed network.
13. The method of claim 12, wherein at least one of splitter ratios
of the plurality of splitters or lengths of the plurality of fixed
line lengths are tuned to provide the greater amplitude taper to
the second illumination function as compared to the first
illumination function.
14. The method of claim 10, wherein the amplitude differences are
imparted to the signal pairs via a plurality of quadrature hybrid
couplers of the feed network.
15. The method of claim 14, wherein the antenna array supports
radio frequency (RF) signals over a range of RF frequencies,
wherein the range of RF frequencies includes the first frequency
and the second frequency.
16. The method of claim 15, wherein each of the plurality of
quadrature hybrid couplers is tuned to a designated frequency
within the range of RF frequencies.
17. The method of claim 16, wherein the designated frequency
comprises a mid-frequency within the range of RF frequencies.
18. The method of claim 10, wherein the first illumination function
is to provide a first radiation pattern via the antenna array, and
wherein the second illumination function is to provide a second
radiation pattern via the antenna array, wherein the first
radiation pattern and the second radiation pattern provide a
substantially identical coverage footprint for a range of angles
from three degrees above the horizon to three degrees below the
horizon for a deployment of the antenna array.
Description
FIELD OF THE DISCLOSURE
The present disclosure relates generally to antenna systems, and
more particularly to matching service footprints of different
component frequencies.
BACKGROUND
A base station antenna designed for broadband mobile communications
networks may include one or more arrays, each array comprising a
plurality of radiating antenna elements, and the arrays being
capable of supporting one or multiple spectrum bands. The antenna
elements of each array are connected to a radio frequency (RF)
beamforming network (also referred to as a RF distribution network
or a RF feed network), which is designed to distribute RF power to
the antenna elements, when considering a signal for transmission
via the array. The antenna elements of the array are typically
arranged in a vertical plane and designed to create a relatively
narrow radiation pattern beam in the elevation plane (of between 5
and 15 degrees, for example). Phase shifters may also be used
between the RF feed network and the antenna elements. The phase
shifters are used to impart a linearly varying phase slope along
the antenna array and hence vary the boresight direction of the
radiated pattern in the elevation plane. This is known as variable
electrical tilt (VET) in order to control and optimize the cellular
network coverage and interference parameters.
Mobile telecommunications operators deploy base station equipment
which may include a baseband apparatus, radio equipment, and
antennas. The radio equipment has been traditionally designed to
provide RF signals for only one RF band, which in turn is connected
to one or more radiating arrays of the base station antenna. For
multiple RF bands at a base station site, multiple radios are
connected to correspondingly multiple antennas. Each radio plus
antenna combination may have the capability to vary the antenna
beam tilt associated with the RF band connected to the antenna, for
network design optimization purposes.
SUMMARY
In one example, the present disclosure provides an antenna system
that includes a first phase shifting network to output a first set
of component signals from a first component signal having a first
frequency, a second phase shifting network to output a second set
of component signals from a second component signal having a second
frequency, where the first frequency is less than the second
frequency, and an antenna array comprising a plurality of antenna
elements coupled to the first phase shifting network and to the
second phase shifting network. In one example, the first phase
shifting network is to impart a first tilt angle to a first
radiation pattern of the antenna array for the first set of
component signals, and the second phase shifting network is to
impart a second tilt angle to a second radiation pattern of the
antenna array for the second set of component signals. The antenna
system may also include a single variable electrical tilt
controller to control the first phase shifting network and the
second phase shifting network, where the single variable electrical
tilt controller is configured to maintain a ratio between the first
tilt angle and the second tilt angle, where the first tilt angle is
greater than the second tilt angle, and where the ratio of the
second tilt angle to the first tilt angle is less than one.
In another example, the present disclosure provides an antenna
system that includes an antenna array comprising a plurality of
antenna elements, a first baseband beamforming unit, to apply a
first plurality of precoding weights to a first set of component
signals associated with a first frequency, and a second baseband
beamforming unit, to apply a second plurality of precoding weights
to a second set of component signals associated with a second
frequency, where the first frequency is less than the second
frequency. In one example, the first plurality of precoding weights
is to impart a first tilt angle to a first radiation pattern of the
antenna array for the first set of component signals, the second
plurality of precoding weights is to impart a second tilt angle to
a second radiation pattern of the antenna array for the second set
of component signals, and the first plurality of precoding weights
and the second plurality of precoding weights are configured to
maintain a ratio between the first tilt angle and the second tilt
angle, where the first tilt angle is greater than the second tilt
angle, and where the ratio of the second tilt angle to the first
tilt angle is less than one. In another example, the first
plurality of precoding weights is to impart a first beamwidth to a
first radiation pattern of the antenna array for the first set of
component signals, the second plurality of precoding weights is to
impart a second beamwidth to a second radiation pattern of the
antenna array for the second set of component signals, and the
first plurality of precoding weights and the second plurality of
precoding weights are configured to maintain the first beamwidth
and the second beamwidth to provide a same mainbeam far-field
radiation pattern for both the first radiation pattern and the
second radiation pattern.
In yet another example, the present disclosure provides an antenna
system that includes an antenna array comprising a plurality of
antenna elements and a feed network to: create signal pairs from an
input signal having at least a first component signal of a first
frequency and a second component signal of a second frequency,
impart frequency-dependent phase differences to the signal pairs,
and impart amplitude differences to the signal pairs, the amplitude
differences of each signal pair dependent upon the
frequency-dependent phase differences of the signal pair, the
signal pairs comprising antenna element drive signals of the
plurality of antenna elements of the antenna array, where the
antenna element drive signals provide a first array illumination
function for the first frequency and a second array illumination
function for the second frequency, where the second frequency is
greater than the first frequency, and where the second illumination
function has a greater amplitude taper compared to the first
illumination function.
BRIEF DESCRIPTION OF THE DRAWINGS
The teaching of the present disclosure can be readily understood by
considering the following detailed description in conjunction with
the accompanying drawings, in which:
FIG. 1A shows a graph of the elevation radiation patterns for RF
signals at 750 MHz and 880 MHz, of an antenna system with dual-band
radios being connected to the same antenna array and tilted at a 9
degree tilt angle;
FIG. 1B illustrates a plot of the approximate coverage service area
served by each RF component signal of FIG. 1A;
FIG. 2A illustrates a graph of the elevation radiation patterns for
RF signals at 750 MHz and 880 MHz, of a commercially available
antenna system with dual-band radios and the 750 MHz signal tilted
at 11 degrees and the 880 MHz signal tilted at 9 degrees;
FIG. 2B illustrates a plot of the approximate coverage service area
served by each RF component signal of FIG. 2A;
FIG. 3 illustrates a first example antenna system, an antenna
element amplitude graph, a chart of relative phase at each antenna
element for two different RF signal frequencies, and a graph of the
resulting far field radiation patterns as a function of elevation
angle for the two different RF signal frequencies;
FIG. 4 illustrates a table of values representing the outputs of RF
propagation simulations of how well correlated the service coverage
footprints are aligned between signals carrying 750 MHz and 880 MHz
communications services using a conventional antenna system;
FIG. 5 illustrates a second example antenna system, an antenna
element amplitude graph for a first RF signal frequency, and a
graph of the resulting far field radiation patterns as a function
of elevation angle for the first RF signal frequency;
FIG. 6 illustrates the second example antenna system, an antenna
element amplitude graph for a second RF signal frequency, and a
graph of the resulting far field radiation patterns as a function
of elevation angle for two RF signal frequencies; and
FIG. 7 illustrates an example antenna system with digital
beamforming, in accordance with the present disclosure.
To facilitate understanding, identical reference numerals have been
used, where possible, to designate identical elements that are
common to the figures.
DETAILED DESCRIPTION
Examples of the present disclosure include antenna systems for
multi-band cellular base station deployment. In particular,
examples of the present disclosure provide elevation plane
radiation patterns for different RF component frequencies such that
the resultant service cell range or footprint for different RF
component frequencies are substantially similar, over a range of
variable mainbeam tilt angles. Examples of the present disclosure
may be used with baseband carrier aggregation features in access
technologies such as Long Term Evolution (LTE). For instance,
aligning component carrier footprints with each other maximizes
carrier aggregation efficiency and also avoids unnecessary
inter-cell interference in the wider cellular network.
Dual band (and multi-band) technologies simultaneously support
proximate RF bands through one radio equipment. For example, in the
United States, the 700 MHz and 850 MHz bands provide macrocellular
service and have been supported using separate 700 MHz and 850 MHz
radio equipment, connected to separate antennas, or at least
separate antenna connections on a multi-port antenna. Dual-band
radios allow a reduction in equipment or "box" count for mobile
operators at base station sites. Furthermore, 4G radio access
technologies and beyond such as Long Term Evolution (LTE) allow
bonding of two or more RF channels from different bands together to
support higher traffic capacity solutions, referred to as carrier
aggregation (CA). RF bands such as 850 MHz in the United States are
being re-purposed from supporting 3G access technologies to 4G/LTE
access technology, which then permits carrier aggregation to be
used. Cellular networks using carrier aggregation can be optimized
by ensuring the service coverage footprints of the component RF
bands are aligned or overlaid on top of each other. This could be
achieved by varying the beam tilt of the antenna supporting each RF
band. If one band has less coverage extent than another band in a
dual-carrier, carrier aggregation example, then this may result in
the benefits of dual-carrier, carrier aggregation not being
available to all mobile subscribers.
Dual-band radios transmit and receive signals in the two RF bands,
together being connected to the same antenna. The antenna may
include an array of antenna elements designed for wideband
operation in that the antenna is able to support both bands. For
example an antenna capable of operating in the range 698-960 MHz
can support both the 700 MHz and 850 MHz RF bands used in the USA.
The antenna array may have a directional radiation beam in the
elevation plane, which may be between 5 and 15 degrees beamwidth,
depending upon the length of the antenna array and frequency. The
radiation beam may also be tilted in the elevation plane to
optimize for coverage and minimize inter-cell interference.
FIG. 1A shows a graph 100 of the elevation radiation patterns for
RF signals at 750 MHz (solid plot) and 880 MHz (dotted plot), of a
commercially available antenna system with dual-band radios being
connected to the same antenna array and tilted at a 9 degree tilt
angle. FIG. 1A is presented in Cartesian coordinates from elevation
angles -30 degrees (pointing toward the sky) to +30 degrees
(pointing toward the ground). The vertical line 103 in FIG. 1A is a
1 degree inclination angle representing the angle from the base
station antenna at 25 m above ground to the edge of the cell
service coverage area at 1.4 km distance away from the base station
site, which is typical of many cell sites.
FIG. 1B illustrates a plot 110 of the approximate coverage service
area served by each RF component. The intersection of the 750 MHz
and 880 MHz radiation patterns in FIG. 1A at this 1 degree
inclination angle reveals a 4 dB difference in radiated signal
strength at the cell edge between the two RF bands, 1.7 km away.
Despite the 880 MHz and 750 MHz signals having the same beam tilt,
the 750 MHz signal component will arrive at the cell edge some 4 dB
stronger than the 880 MHz signal (e.g., assuming no other
propagation differences are accounted for). Notably, the elevation
beamwidth narrows with higher frequencies since beamwidth is an
inverse function of array length (which is fixed in this case) and
frequency. This may result in a degraded 750 MHz signal component
towards the cell edges due to an increase in inter-cell
interference. The 750 MHz signal could be re-optimized for
inter-cell interference by using a more aggressive tilt angle, but
this may introduce a reduced signal component footprint at 880
MHz.
One solution to provide an antenna array capable of supporting
multiple RF bands which has an elevation radiation pattern over a
range of angles which does not change, or changes minimally, with
frequency, involves using a diplexer to separate the different band
signals (e.g., 750 MHz and 880 MHz) from a dual-band radio and then
routing the frequency dependent component signals to separate
antennas or antenna ports which allow the elevation patterns to be
tilted independently. FIGS. 2A and 2B illustrate the result where
the 750 MHz signal component is tilted at 11 degrees rather than 9
degrees. In particular, FIG. 2A shows a graph 290 of the elevation
radiation patterns for RF signals at 750 MHz (solid plot) and 880
MHz (dotted plot), of a commercially available antenna system with
dual-band radios and the 750 MHz signal component is tilted at 11
degrees and the 880 MHz signal tilted at 9 degrees. The vertical
line 203 in FIG. 2A is a 1 degree inclination angle representing
the angle from the base station antenna at 25 m above ground to the
edge of the cell service coverage area at 1.4 km distance away from
the base station site. FIG. 2B illustrates a plot (299) of the
approximate coverage service area served by each RF component. The
disadvantage of using a diplexer and independent tilt controls is
that it requires managing and calculating two tilt angles, and
possibly having additional antennas at site.
Examples of the present disclosure optimize antenna elevation beam
radiation patterns for use with dual-band or multi-band radios
without the need for separate tilt control of different bands and
without separate antenna elements or antenna arrays for each band.
FIG. 3 illustrates an example antenna system 301, the operations of
which are described in connection with an example of the processing
of an RF signal intended for transmission. A dual-band RF input
signal Vin comprising RF signals in a 700 MHz spectrum band (e.g.,
750 MHz) and an 850 MHz spectrum band (e.g., 880 MHz) is connected
to a first diplexing filter 235 which separates the dual-band RF
signal into two frequency dependent component signals; the first
frequency dependent component signal being associated with the 700
MHz band ("7") and the second frequency dependent component signal
being associated with the 850 MHz band ("8"). The 700 MHz band
frequency dependent component signal is processed via a first RF
feed network 225 (also referred to herein as a "RF distribution
network") and a first phase shifting network comprising eight phase
shifters 310.sub.1-8 producing a set of eight component signals of
the first frequency dependent component signal. The 850 MHz band
frequency dependent component signal is processed via a second RF
feed network 220 and a second phase shifting network comprising
eight phase shifters 320.sub.1-8 producing a series of eight
component signals of the second frequency dependent component
signal. The respective outputs of the series of eight component
signals of the first frequency dependent component signal (700 MHz
band) and the series of eight component signals of the second
frequency dependent component signal (850 MHz), are combined in a
series of eight dual-band diplexing filters 330.sub.1-8. The series
of combined RF signals connected to antenna elements in an antenna
array, in this case one polarized array of a dual-polarized array
of antenna elements (450.sub.1-8).
Each phase shifting network 310.sub.1-8 and 320.sub.1-8 is designed
to impart a variable phase slope function for each respective band
along the antenna array (450.sub.1-8) in order to vary elevation
plane radiated pattern beam tilt. The two phase shifting networks
310.sub.1-8 and 320.sub.1-8 could be independently controlled to
create variable electrical tilt (VET) beam tilts to ensure cell
edge coverage footprints for each band are overlaid with each
other. However, in the present example, a single VET control is
used and the two phase shifting networks are coupled to each other
via a coupling mechanism, e.g., tilt controller 315, where the
coupling is designed to deliver a greater boresight beam tilt angle
for the 700 MHz band signals than the 850 MHz band signals, to
ensure a more optimal cell edge overlay between the two bands. In
one example, the tilt controller 315 may comprise a mechanical
linkage between the two phase shifting networks or an electrical
connection for control signals. The present example maintains a
similar radiated pattern around the -3.degree. to +3.degree. region
of the elevation patterns, regardless of boresight tilt angle of
the component RF frequencies. This particular range of angles
(-3.degree. to +3.degree. region) will generally encompass the cell
edge and most of the cell area for typical macro cell sites,
including undulating terrain and buildings. For example, a base
station cell site having an antenna height above the ground of 25 m
and a cell edge distance from the cell site of 1.4 km over flat
earth equates to an inclination angle of +1 degree. It can also be
shown that the area of the cell within +1.degree. and +3.degree.
inclination represents well over 80% of the cell total area.
The illumination function of the antenna array is shown by the
antenna element amplitude graph 501 which illustrates the relative
amplitudes of component signals 550.sub.1-8 at the respective
antenna elements 450.sub.1-8 from either of the RF distribution
networks 225 or 220, which in this example are essentially the same
feed network. The illumination function of the present example is
essentially invariant with respect to RF frequency. The phase
profiles or relative phase at each antenna element 651.sub.1-8 and
660.sub.1-8, are depicted in the chart 601 for two different RF
signal frequencies of 750 MHz and 880 MHz, respectively. It should
be noted that the signal frequency of 750 MHz may fall within the
700 MHz band (which may range from 698-803 MHz, in one example),
and the signal frequency of 880 MHz may fall within the 850 MHz
band (which may range from 824-894 MHz, in one example). The graph
701 depicts the resulting far field radiation patterns 751 and 760
as a function of elevation angle (over -30 degrees to +30 degrees),
for the two different RF signal frequencies of 750 MHz and 880 MHz,
respectively. The coupled RF phase shifting networks 310.sub.1-8
& 320.sub.1-8 introduce phase delays for each respective
spectrum band such that the radiated patterns are similar over the
angular range -3.degree. and +3.degree.. In FIG. 3, the 880 MHz RF
signal has a beam tilt of 6 degrees and the 750 MHz RF signal has a
beam tilt of around 7 degrees, such that the radiation patterns for
both RF signals are well aligned around the range of angles most
critical for determining cell range and service footprint, in this
case the region 800 highlighted in the graph 701 of FIG. 3 as the
elevation angles between -3 degrees and +3 degrees.
It should be understood that example of FIG. 3 is described for a
portion of the antenna system 301 that includes a first
polarization component array of the dual-polarized array of antenna
elements 450.sub.1-8. Thus, the antenna system 301 may include the
same or similar components for the second polarization component
array (e.g., additional feed/distribution networks, additional
phase shifting networks, additional dual-band diplexing filters,
etc.).
FIG. 4 depicts a table 499 which has values representing the
outputs of RF propagation simulations of how well correlated the
service coverage footprints are aligned between signals carrying
750 MHz and 880 MHz communications services using a conventional
antenna system, as a function of boresight tilt for each RF signal
and with a base station antenna height of 30 m. For example, the
table 499 reveals that if the 750 MHz and 880 MHz RF signals are
both tilted at 10 degrees, there is an 79% goodness metric for
service footprint correlation, measured as the smaller of (a) the
coverage area of the 750 MHz service divided by the coverage area
of the 880 MHz service and (b) the coverage area of the 880 MHz
service divided by the coverage area of the 750 MHz service. The
table 499 reveals that a more optimal correlation can be achieved
if the 750 MHz RF signal was tilted at 12 degrees where a 94%
correlation is achieved. A regression analysis of the data
indicates that an empirical formula for optimal boresight tilt
values can be derived as T.sub.880=0.87.times.T.sub.750, where
T.sub.880 is the tilt value for the 880 MHz RF signals and
T.sub.750 is the tilt value for the 750 MHz signals. Extending this
analysis (not shown) allows a more general empirical formula of the
form T.sub.f=1+(f-800)/800.times.T.sub.800 to be derived where f is
the frequency, T.sub.f is the tilt value for the signals at
frequency f, and T.sub.800 is the tilt value of RF signals at 800
MHz, where 800 MHz is a nominal mid-frequency of the antenna (for
example, the antenna may have a nominal operational range of
698-894 MHz). Other similar antennas, for example having the same
number of elements and approximate array length reveal similar
outcomes and have similar empirical formulas for characterizing
optimal tilt values.
In one example, the formula can be used to determine the desired
coupling behavior between the two phase shifting networks
310.sub.1-8 & 320.sub.1-8 when using a single tit controller
315 (e.g., a VET controller), rather than two independent VET
controls. Antenna height above ground and cell range may also be
taken into account in the empirical formula to allow further
optimization and to ensure cell edges of the respective frequency
bands are overlaid for a range of cell deployments. However, cell
height and cell edge distance tend to correlate, and as such height
and range refinements tend to be less critical, and relatively
insensitive. A single tilt controller 315 for controlling the
respective band dependent phase shifting networks, using a tilt
formula based around the average macro cell site (e.g. 25 m and 1.4
km range in the US), will almost be always more optimal than using
a conventional antenna with respect to having correlated cell edge
overlay between bands.
A second example varies the antenna aperture illumination function
according to frequency in order to maintain a near constant
beamwidth over a range of frequencies. A flat or rectangular
aperture illumination function leads to narrow beamwidth, and a
tapered aperture illumination function leads to wider beamwidth.
Maximum directivity, and hence gain, is often preferred over a
range of tilt angles while minimizing upper sidelobe levels in the
elevation plane, e.g., below 15 dB relative to the mainlobe, since
sidelobes can lead to inter-cell interference.
The second example includes an RF power distribution network which
divides RF power across a number of antenna elements in an antenna
array. The RF distribution network includes hybrid couplers for
converting differentially phased RF signals into a pair of signals
having different (and complementary) amplitudes (e.g., the
amplitude differences are a function of the phase difference).
These signals having varying amplitudes are used to drive antenna
elements in the antenna array as part of the aperture illumination
function. The phase difference of the differentially phased signals
is generated by using RF lines of different lengths, where the
phase difference is frequency-dependent and varies as an inherent
function of frequency.
The second example antenna system 302 is described in reference to
FIG. 5 and in connection with the processing of an RF signal
intended for transmission. An input signal, V at frequency 880 MHz,
having a corresponding wavelength of 340 mm, is connected to an RF
distribution network 200. The signal is first split into two
component signals via a RF splitter 201. The first component signal
is split into third and fourth component signals via an equal power
splitter 202. The second component signal is split into fifth and
sixth component signals via an equal power splitter 203.
The third and fourth component signals are fed to a first
quadrature hybrid coupler 206, where the third component signal is
fed via a first fixed line length 204 (which is approximately 220
mm longer than a line length for the fourth component signal) such
as to impart a phase difference of 128 degrees between third and
fourth component signals at the input ports of the first quadrature
hybrid coupler 206. The first quadrature hybrid coupler 206 is
tuned to a designated frequency within a range of RF frequencies
supported by the array of antenna elements 400.sub.1-8, e.g., a
center or mid-frequency of 810 MHz, and constructed of four
quarter-wavelength .lamda./4 branches of approximately 93 mm line
length as illustrated. The two output signals from the first
quadrature hybrid coupler 206 have vector equations as shown by the
outputs of the first quadrature hybrid coupler 206. These output
equations take account of the phase mismatches associated with
signals at 880 MHz and that the quadrature hybrid coupler 206 in
the present example is tuned to 810 MHz (where an input signal at
810 MHz will result in output signals having a 90 degree phase
difference). The amplitude difference of the two output signals is
dependent upon the phase difference exhibited by the third and
fourth component signals input to the first quadrature hybrid
coupler 206. The first output signal from the first quadrature
hybrid coupler 206 is delayed by a second fixed line length 208
being tuned to impart a 180 degree phase delay at 810 MHz, which is
connected in turn to a two-way equal power RF splitter 210
providing RF drive signals for antenna elements 400.sub.1 and
400.sub.8. The second output signal from the first quadrature
hybrid coupler 206 is not delayed and is connected to a two-way
equal power RF splitter 211 providing RF drive signals for antenna
elements 400.sub.4 and 400.sub.5.
The fifth and sixth component signals are connected to a second
quadrature hybrid coupler 207, where the fifth component signal is
fed via a third fixed line length 205 (which is approximately 200
mm longer than a line length for the sixth component signal) such
as to impart a phase difference of 149 degrees between fifth and
sixth component signals at the input ports of the second quadrature
hybrid coupler 207. The second quadrature hybrid coupler 207 is
also tuned to a center frequency of 810 MHz and constructed of four
quarter-wavelength .lamda./4 branches of approximately 93 mm line
length as illustrated. The two output signals from the second
quadrature hybrid coupler 207 have vector equations as shown by the
outputs of the second quadrature hybrid coupler 207. These output
equations take account of the phase mismatches associated with
signals at 880 MHz and that the second quadrature hybrid coupler
207 in the present example is tuned to 810 MHz (where an input
signal at 810 MHz will result in output signals having a 90 degree
phase difference). The amplitude difference of the two output
signals is dependent upon the phase difference exhibited by the
fifth and sixth component signals input to the second quadrature
hybrid coupler 207. The first output signal from the second
quadrature hybrid coupler 207 is delayed by a fourth fixed line
length 209 being tuned to impart a 180 degree phase delay at 810
MHz, which is connected in turn to a two-way equal power RF
splitter 212 providing RF drive signals for antenna elements
400.sub.2 and 400.sub.7. The second output signal from the second
quadrature hybrid coupler 207 is not delayed and is connected to a
two-way equal power RF splitter 213 providing RF drive signals for
antenna element 400.sub.3 and 400.sub.6.
A phase shifting network of variable phase shifters 300.sub.1-8 is
disposed between the outputs of the final stage of RF splitters
(210, 211, 212, 213) and the array of antenna elements 400.sub.1-8
to provide variable electrical tilt (VET) control of the elevation
plane beam. The graph 502 in FIG. 5 depicts the relative amplitudes
of the RF drive signals 500.sub.1-8 corresponding to each antenna
element 400.sub.1-8 and hence illustrates the illumination function
of the antenna aperture, which in this example has a symmetric
taper where amplitude increases from the edge elements to the
center elements. The graph 702 depicts the far field radiation
pattern 700 as a function of elevation angle (over -30 degrees to
+30 degrees) representative of the aperture illumination function
500.sub.1-8 with no additional phase shifts applied to the elements
from phase shifting network 300.sub.1-8. The radiated pattern has a
vertical beamwidth of about 7 degrees (e.g., half-power beamwidth
(HPBW)) and upper sidelobe levels of less than -17 dB.
FIG. 6 depicts the same antenna system 302 as FIG. 5, but for an
input signal, V, at 750 MHz being processed. When a signal of 750
MHz, having a corresponding wavelength of 400 mm is input to the RF
distribution network 200, the fixed line lengths 204 and 205 create
phase differentials across the inputs to the first quadrature
hybrid coupler 206 and across the inputs to the second quadrature
hybrid coupler 207, which are different phase differentials to
those in the preceding example of a signal of 880 MHz being
processed. These phase differentials in turn create signal
amplitudes at the outputs of the first quadrature hybrid coupler
206 and at the outputs of the second quadrature hybrid coupler 207,
which are different signal amplitudes to those in preceding example
of a signal of 880 MHz being processed. The outputs of the
quadrature hybrid couplers 206 and 207 are phase corrected by fixed
line lengths 208 and 209 and further split by equal power RF
splitters (210, 211, 212, 213) which create an illumination
function across the array of elements 400.sub.1-8, as previously
described.
The resulting illumination function of the aperture for an input
signal of 750 MHz is shown in the graph 503 as RF signal amplitudes
510.sub.1-8. The illumination function in FIG. 6 is much more
rectangular (e.g., flat/non-tapered) than the illumination function
500.sub.1-8 associated with the processing of a signal at 880 MHz
by the RF distribution network 200. In other words, the graph 502
illustrates a greater amplitude taper of the illumination function
as compared to the illumination function of the graph 503. However,
it should be noted that the illumination function 510.sub.1-8 is
not entirely rectangular in that the two center elements have a
slightly stronger amplitude than the other elements. The graph 703
depicts the far field radiation pattern 710 as a function of
elevation angle (over -30 degrees to +30 degrees) representative of
the aperture illumination function 510.sub.1-8 with no additional
phase shifts applied to the elements from phase shifting network
300.sub.1-8. The radiated pattern has a vertical beamwidth of about
7 degrees and upper sidelobe levels of less than -17 dB. However,
the mainbeam radiated pattern is almost identical to the radiated
pattern for a signal of 880 MHz (shown in graph 703 as far field
radiation pattern 700). In addition, the RF distribution network
200 provides mainbeam patterns that remain relatively invariant
(e.g., within 1 dB difference at any tilt angle of interest, such
as +/-3 degrees above and below the horizon) over a range of
frequencies from 700 MHz to 960 MHz. As such, the example of FIGS.
5 and 6 is suitable for triple-band LTE carrier aggregation (CA)
applications.
It should be noted that at least one of splitter ratios of the
plurality of splitters or lengths of the plurality of fixed line
lengths are tuned to provide the greater amplitude taper to the
illumination function for the higher band signal (e.g., 880 MHz) as
compared to the illumination function for the lower band signal
(e.g., 750 MHz). Thus, it should be understood that in other,
further, and different examples, the array of antenna elements
400.sub.1-8 may support a different range of RF frequencies and/or
the frequency bands of the component signals of the input signal
may be different. In addition, the quadrature hybrids 206 and 207
may have different dimensions and may be tuned to different
frequencies. Alternatively, or in addition, the feed network 200
may utilize different splitter ratios and/or line lengths so as to
provide the greater amplitude taper to the radiation pattern for a
higher-band component signal as compared to a lower-band component
signal of an input signal.
The foregoing examples illustrate the principles of the present
disclosure with regard to passive RF distribution and phase
shifting networks. However, additional examples of the present
disclosure also include baseband beamforming where amplitude and
phase weights are generated at baseband associated with the
respective component RF frequencies, such that the resultant
elevation radiation patterns for different RF frequency components
are aligned with each other over the range of elevation angles that
are used to determining the cell range. The alignment can be
achieved through adjusting the respective tilt angles for two
component frequency signals in a coordinated manner (e.g., using
the formulas described above in connection with the examples of
FIGS. 3 and 4) and/or via adjusting the beamwidth, or illumination
function taper, as described in connection with the example of
FIGS. 5 and 6.
FIG. 7 illustrates an example antenna system 303 where digital
beamforming is applied. A first baseband signal intended for
transmission at an RF band at 700 MHz is split into component
baseband signals and vector weights (p1 to p4) are applied to the
component baseband signals of the first baseband signal at a first
baseband beamforming unit 10. A second baseband signal intended for
transmission at an RF band at 850 MHz is split into component
baseband signals and vector weights (r1 to r4) are applied to the
component baseband signals of the second baseband signal at a
second baseband beamforming unit 20. The baseband component signals
from the first baseband signal and the baseband component signals
from the second baseband signals are vector summed using summing
functions 30.sub.1-N (broadly a "vector summing unit"). The
composite baseband component signals are then upconverted from
baseband to RF and amplified via baseband-to-RF conversion unit 40.
The resulting RF signals are then fed to N antenna elements
62.sub.1-N, via an analog feed network 50. The analog feed network
50 is optional and serves to distribute four composite RF signals
(drive signals) to antenna elements 62.sub.1-N, e.g., 8 antenna
elements. In one example, an optional analog phase shifting network
64.sub.1-N can be used for additional tilt control.
While the foregoing describes various examples in accordance with
one or more aspects of the present disclosure, other and further
example(s) in accordance with the one or more aspects of the
present disclosure may be devised without departing from the scope
thereof, which is determined by the claim(s) that follow and
equivalents thereof. For instance, for illustrative purposes the
foregoing examples are described primarily with respect to transmit
signals. However, it should be understood that the foregoing
examples are equally applicable to receive signals with respect to
the same or similar frequency band and/or frequency component
signals.
Various aspects of the present disclosure may also include the
following:
Examples of the present disclosure include antenna systems for
multi-band cellular base station deployment having elevation plane
radiation patterns for different RF component frequencies such that
the resultant service cell range or footprint for different RF
component frequencies are substantially similar.
In one example a phased array antenna system may comprise a
plurality of antenna elements, the antenna system supporting RF
signals over a range of RF frequencies and creating a directive
beam in the elevation plane having a single variable electrical
tilt control means, characterized in that the range of the service
area coverage footprint formed for a first frequency component
signal of the RF signal is substantially identical to the range of
the service area coverage footprint formed for different frequency
component signals of the RF signal, for a range of electrical tilt
angles associated with the first frequency component signal of the
RF signal. In one example, the antenna system may be connected to a
radio system for transmission and reception of RF signals with
component signals from at least two RF spectrum bands. In addition,
in one example, the radio system may be connected to a baseband
unit providing carrier aggregation features in at least two RF
spectrum bands supported by the radio system.
In one example, the elevation plane radiation pattern is
substantially invariant over the range of elevation angles +/-3
degrees with respect to the horizon for a range of frequencies
supported by the phased array antenna system. In addition, in one
example, the majority of the mainbeam elevation plane radiation
pattern is substantially invariant across a range of frequencies
supported by the phased array antenna system.
In one example, the phased array antenna system comprises a first
diplexor for splitting an RF signal into at least a first frequency
dependent component signal and a second frequency dependent
component signal, where the phased array antenna system is further
to split the first frequency dependent component signal into
further component signals and to connect to a first phase shifting
network for delivering variable electrical tilt for the first
frequency dependent component signal, where the phased array
antenna system is further to split the second frequency dependent
component signal into further component signals and to connect to a
second phase shifting network for delivering variable electrical
tilt for the second frequency dependent component signal, the
phased array antenna system further comprising a coupling mechanism
between first and second phase shifting networks.
In one example, the antenna system includes a feed network which
creates pairs of component signals, each pair of component signals
having a phase difference dependent upon RF frequency and being
converted into pairs of component signals characterized in having
amplitude differences, the amplitude differences being dependent
upon the component signal pair phase differences, which are used as
drive signals for the antenna array illumination function. In such
an example, the antenna element drive signals may have an array
illumination function which has increasing amplitude taper with
increasing RF frequency.
In one example, the radio system comprises an active beamforming
network, and the beamforming is developed at baseband. Example
embodiments may also include methods of operating any of the
example phased array antenna system described above for transmit
signals, receive signals, or both transmit signals and receive
signals.
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